1. Field of the Invention
The present invention generally relates to an oscillation circuit, more particularly, to an oscillation circuit having less current consumption and a method of obtaining an oscillation signal.
This application is a counterpart of Japanese patent application, Ser. No. 180259/1998, filed Jun. 26, 1998, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
In general, the operation of a semiconductor integrated circuit, such as a microcomputer, is controlled by a clock signal. Basically, the operating speed of an entire system which includes the microcomputer depends upon a frequency of the clock signal (clock rate). A crystal oscillation circuit having a CMOS type inverter is well known as an oscillation circuit which produces such a clock signal.
An example of a conventional oscillation circuit is shown in FIG. 5.
The oscillation circuit has oscillation terminals OSC0 and OSC1, a crystal resonator 1 (or a quartz resonator 1), a CMOS inverter 2 having a P-channel transistor TR1 and an N-channel transistor TR2, a set of capacitors C1 and C2, and a feedback resistor FR. Input and output terminals of the CMOS inverter 2 are respectively connected to the oscillation terminals OSC0 and OSC1. The oscillation circuit further includes current limiting resistors LR1 and LR2 which are connected to the CMOS inverter 2. The oscillation terminals OSC0 and OSC1, the feedback resistor FR, the CMOS inverter 2, and the current limiting resistors LR1 and LR are located inside a chip and the remaining elements are positioned outside the chip.
The current limiting resistors LR1 and LR2 limit an operating current to an appropriate amount while oscillating. Thus, a current consumption of the oscillation circuit can be reduced. Especially in the case where the resonant frequency of the crystal resonator 1 is relatively low, the current consumption can be substantially decreased by increasing a resistance of the current limiting resistors LR1 and LR2.
However, in the conventional oscillation circuit, a gain of the oscillation circuit decreases with increasing resistance of the resistors LR1 and LR2. Therefore, if a relatively large resistance is simply applied to the oscillation circuit having the crystal resonator 1 of a relatively high resonant frequency, it is not easy to achieve oscillation just after a power supply voltage is applied thereto. In fact, in this oscillation circuit, there is a possibility that oscillation is not achieved at all. On the other hand, if a relatively high resistance is simply applied to the oscillation circuit, there is a possibility that the oscillation circuit, which has the crystal resonator 1 of a relatively low resonant frequency, will exhibit an abnormal oscillation.
There are two ways to overcome this problem. One way is to prepare two kinds of oscillation circuits, one of which has resistors of relatively small resistance for a high frequency oscillation and the other of which has resistors of relatively large resistance for a low frequency oscillation. However, this reduces the mass production efficiency.
The other way is the so called mask option where extra masks are prepared to change resistors. However, this also reduces mass production efficiency.
Consequently, there has been a need for an improved oscillation circuit that may correctly oscillate at both low and high frequencies.